Enhancement of Proton Conductivity in Nonporous Metal–Organic Frameworks: The Role of Framework Proton Density and Humidity

Owing
to their inherent pore structure, porous metal–organic
frameworks (MOFs) can undergo postsynthetic modification, such as
loading extra-framework proton carriers. However, strategies for improving
the proton conductivity for nonporous MOFs are largely lacking, although
increasing numbers of nonporous MOFs exhibit promising proton conductivities.
Often, high humidity is required for nonporous MOFs to achieve high
conductivities, but to date no clear mechanisms have been experimentally
identified. Here we describe the new materials MFM-550­(M), [M­(HL1)], (H4L1 = biphenyl-4,4′-diphosphonic
acid; M = La, Ce, Nd, Sm, Gd, Ho), MFM-550­(Ba), [Ba­(H2L1)], and MFM-555­(M), [M(HL2)], (H4L2 = benzene-1,4-diphosphonic acid; M = La, Ce, Nd, Sm,
Gd, Ho), and report enhanced proton conductivities in these nonporous
materials by (i) replacing the metal ion to one with a lower oxidation
state, (ii) reducing the length of the organic ligand, and (iii) introducing
additional acidic protons on the MOF surface. Increased framework
proton density in these materials can lead to an enhancement in proton
conductivity of up to 4 orders of magnitude. Additionally, we report
a comprehensive investigation using in situ 2H NMR and
neutron spectroscopy, coupled with molecular dynamic modeling, to
elucidate the role of humidity in assembling interconnected networks
for proton hopping. This study constructs a relationship between framework
proton density and the corresponding proton conductivity in nonporous
MOFs, and directly explains the role of both surface protons and external
water in assembling the proton conduction pathways.